MP Distributor Tuning Tutorial

[DrCharles]

That slew rate will retard the timing up to 2 degrees per 1000 RPM and you can't see that with a timing light.

Old thread or not... You can't see the additional box delay on a distributor machine, because the box is not in the circuit. But you certainly can see it with a timing light. The light flashes when the spark actually happens, since it's triggered by the current passing down the plug wire... which will be a couple of degrees later (retarded) by the box.
We may not notice it except to observe a little less advance at higher RPM than expected. But it's definitely included in the circuit from reluctor tooth to plug ;)

 

[mbaird]

I didn't read the entire thread but noted his concern about the excessive centrifugal advance at 4000+. Not sure if he considered the loss of timing due to slew rate.

 

[Mattax]

You can't see the additional box delay on a distributor machine, because the box is not in the circuit.

On the distributor machine it depends on what is being used for a trigger box. The box for use with the older Sun machines is pretty supposed to relatively slow. That's what I used. Others have made their own with some faster devices. I think YR is using an Allen machine.

 

[DrCharles]

True... and don't forget that timing lights can ALSO have retard with increasing rpm

 

[yellow rose]

I'm testing the boxes on my machine and you can see it. If you do it with a timing light, is it the light, the box, or both?

I'm going to make a practice of not doing a distributor without the box. So far, I'm seeing a difference between boxes of the same part number.

 

[DrCharles]

Interesting! How much of a difference between same-model boxes?

 

[yellow rose]

Interesting! How much of a difference between same-model boxes?

So far, most of the same part number boxes have the same amount of total retard, but where the retard starts, and where it changes with RPM can be quite different.

Don't have my notes right in front of me because I'm working on a pair of tunnel ram carbs, but off the top of my head one box started to retard at 3100 and the same part number box didn't start until 3800. And both boxes retarded from there at different RPM.

I have no idea why that is. I may have to call Tuner and bend his ear and hear his thoughts.

Plus, I owe Tuner lunch, so when I buy him lunch I'll be taking notes!!!

 

[RLF Cuda]

Hmmmmm. I've got to get my machine up and running to test all this. I didn't see how old the thread was!!! Haven't been around much so I'm trying o catch up when I can.

I am rereading this thread as I want to change the mechanical advance on my 340 with Edelbrock heads. I have the original Mopar electronic distributor with the slotted t-bar advance mechanism and have to weld or limit the travel of the advance plate. What I need is a guide for the length of the slot vs the number of degrees advance in the distributor. Has anyone done this measurement?

BTW, the vacuum advance only comes into play during cruise when vacuum is high. Vacuum advance doesn't work at idle if you use the ported slot in the carburetor. Vacuum advance is important for fuel economy and proper combustion at cruise conditions. You generally want about 50° total advance (maybe more) to optimize combustion and fuel economy, and prevent fuel washing of the cylinder bore which reduces engine life. See a number of articles in Mopar Action for details. Properly set up, my 340 got 19 MPG on a cruise of 60 miles. The combination at the time was stock 340 heads matched to the intake (AirGap) and throat ported. 284/484 cam, A727, 3.55 8 3/4 rear, 245-60x15 BFGs.

 

[DrCharles]

Does this help?

 

[Mattax]

I am rereading this thread as I want to change the mechanical advance on my 340 with Edelbrock heads. I have the original Mopar electronic distributor with the slotted t-bar advance mechanism and have to weld or limit the travel of the advance plate. What I need is a guide for the length of the slot vs the number of degrees advance in the distributor. Has anyone done this measurement?

BTW, the vacuum advance only comes into play during cruise when vacuum is high. Vacuum advance doesn't work at idle if you use the ported slot in the carburetor. Vacuum advance is important for fuel economy and proper combustion at cruise conditions. You generally want about 50° total advance (maybe more) to optimize combustion and fuel economy, and prevent fuel washing of the cylinder bore which reduces engine life. See a number of articles in Mopar Action for details. Properly set up, my 340 got 19 MPG on a cruise of 60 miles. The combination at the time was stock 340 heads matched to the intake (AirGap) and throat ported. 284/484 cam, A727, 3.55 8 3/4 rear, 245-60x15 BFGs.

Don't limit the advance.
Shorten the slot on the inside like Rockable did here.
Then its pretty much a pre-CAP advance.
Set initial in the range of a per-CAP high performance LA. 10-15 around 650 rpm.
If the engine is modified, use the displacement and duration as a starting point. Refer to this chart. Initial Timing - Barry Grant at archive.org